This invention relates to the art of precision dispensing of small quantities of fluid, for example, viscous material such as adhesive on circuit boards and other surfaces, and more particularly to a new and improved precision dispensing apparatus and method for accomplishing the foregoing.
One area of use of the present invention is dispensing small quantities of viscous material, although the principles of the present invention can be variously applied to dispensing other types of fluids. Systems and methods for the deposition of drops of adhesive, conductive epoxy, soldering paste, and other viscous fluids at discrete locations on various surfaces are used extensively in modern manufacturing techniques. Such systems include a fluid dispenser and computer controlled apparatus for directing the fluid dispenser to precise locations on any surface. The latter can include a gantry arrangement for X-Y axis movement of the dispenser, a moving table for use with a stationary dispenser and split axis systems for moving the table in one axis and the dispenser in the other.
One form of prior art precision dispenser is a time-pressure dispensing head wherein a pulse of controlled air pressure and time duration forces an amount of adhesive through a dispensing needle. This dispenser is the least costly but the least accurate with respect to the dispensed volume of adhesive. However, the time-pressure dispensing head may be fitted with valve and metering enhancements for improved control. Even so, changes in viscosity of the fluid being dispensed can adversely affect performance.
Another form of prior art precision dispenser is a piston pump dispensing head wherein adhesive flows into an accurately sized chamber, and a measured amount of adhesive is forced by a piston from the chamber through a needle-like dispensing tip. This dispenser is classified as a positive-displacement head. A highly accurate amount of adhesive is dispensed because the chamber contains a known amount of material and the positive displacement is highly controlled. This adhesive dispensing system is virtually unaffected by changes in viscosity of the fluid being dispensed. Examples of piston pump dispensing heads are illustrated in U.S. Pat. Nos. 4,572,103 and 4,941,428.
Still another form of prior art precision dispenser is the screw dispensing head, also called the Archimedes metering valve, in which a motorized auger screw controls the amount of adhesive dispensed. As the screw rotates within a cylinder of adhesive, an accurate amount is forced out through the needle-like dispensing tip. This dispenser offers continuous material feed and fast activation. The adhesive metering valve is somewhat affected by changes in adhesive viscosity, but not to the extent of the time-pressure dispenser. Precise control of the start/stop motion of the screw is an essential requirement for volumetric accuracy. In addition, the amount of material delivered by the Archimedes metering valve is determined by the volume defined by the space between the screw threads and the surface of the chamber in which the screw rotates. An example of the foregoing is illustrated in U.S. Pat. No. 5,564,606.